Photovoltaic devices operate on the basis of charge-separating interfacial topologies
that can be either planar junction (e.g., ci-Si and thin-film inorganic semiconductors) or so-called bulk heterojunction (e.g., OPV). We believe that success in developing economically viable, earth-abundant
solar cells will require innovative approaches and concepts. Our team found that
cooperation between the heterojunction and the homojunction in the hetero-homo dual
junctions (HHDJ) device model enables good performance for CdTe (Cadmium Telluride)
and CIGS (Copper Indium Gallium Selenide) solar cells. In order to test the proposed
approach, we are pursuing the following two types of solar cells:

Zinc phosphide (Zn3P2) is an earth-abundant semiconductor possessing many properties of an ideal absorber
layer for solar cells: long minority carrier diffusion length, high visible light
absorption, and a band gap near 1.5 eV. We synthesize Zn3P2 thin films by close-spaced sublimation.

Cu2ZnSnS4 (CZTS) films made by Spray Pyrolysis

Cu2ZnSnS4 (CZTS) is an earth-abundant semiconductor comprising of non-toxic elements. It has
a band gap of ~1.5 eV and an absorption coefficient of 104 cm-1. Recent research outside of UT has yielded CZTS devices with up to ~11 % efficiency.
The SEM image above shows single-phase CZTS thin films fabricated by ultrasonic spray
pyrolysis.

CZTS nanoparticles made by colloidal synthesis

Literature “one pot” synthesis
UT’s injection method

We have also developed colloidal synthesis approaches to making CZTS nanocrystals,
in an effort to produce starting material with improved stiochiometry control, and
to investigate potential bulk homojunction characteristics. UT’s recent work on synthesis
shows significant improvement in size uniformity, as seen above.

FeS2 Nanocrystals for Thin Film Photovoltaics

Iron sulfide (FeS2), also known as pyrite, offers another promising though challenging route to affordable
solar electricity generation. The bandgap energy of FeS2 is ~1.0 eV, falling very close to the ideal bandgap range of 1.1-1.5 eV for a single-junction
PV device. Although many groups have studied FeS2, challenges related to material defects have yielded only relatively low PV conversion
efficiency of < 3%. Our work seeks to establish very high quality nanocrystal quality
of pyrite in order to use as a building block for highly-controlled thin film stoichiometry
and defect density. The above graphs show cubic pyrite and the corresponding optical
absorption spectrum.

In2S3 window layer and all-sprayed devices

Solutions containing semiconductor nanoparticles and molecular semiconductor precursors
can be processed into films by spraying, dipping, or printing. Here we shows a cross-sectional
SEM of a TEC 15/In2S3/CuInS2 device prepared by spraying (left) and a calculated band diagram for the layers arranged
in two different ways. In2S3 is a non-toxic potential replacement for CdS in several PV devices.